1. Field of the Invention
The present invention relates to an offset correction circuit suitable for a current to voltage conversion circuit that converts an output current from a light emitting element into a voltage.
2. Description of the Related Art
Optical combinations means are known that transmit and receive data via a medium such as light, e.g. infrared rays, for example, as generally exemplified by a remote control for a television set. In such communication means, the light emitting element in the optical transmitter circuit emits data "1" or "0" while the light receiving element in the optical receiver circuit receives the transmitted data according to the output signal of the light receiving element. Then the output signal of the light receiving element is converted into data. FIG. 3 is a diagram illustrating a conventional current to voltage conversion circuit arranged in an optical receiver circuit to convert an output current from a light receiving element into a voltage.
Referring to FIG. 3, the differential amplifier 1 has its positive input terminal connected to the reference voltage source 2 and its negative input terminal connected to the light receiving element 3. A resistor is connected between the output terminal and the negative input terminal. The output current of the differential amplifier 1 is fed back via the resistor 4 to maintain the differential amplifier 1 in equilibrium so that the entire output current of the light receiving element 3 flows through the resistor 4. When receiving light, the light receiving element 3 supplies the output current I1. When the output current I1 flows through the resistor 4, the output voltage Vout (=I1.times.R+Vref, where R is the resistance of the resistor 4) occurs at the output terminal OUT due to a voltage drop across the resistor 4. The output current I1 depends on the light intensity, thus resulting in variations in the output voltage Vout at the output terminal OUT. As a result, an output signal representing "1"or "0" is produced according to the receive data from the output terminal OUT.
The IrDA infrared radiation transmission standards stipulate that the intensity of an effective received light for light reception range from 16 .mu.W to 20 mW in order to detect light with a micro power. Where a micro power light is received with the current to voltage conversion circuit of FIG. 3, the resistance of the resistor 4 is set to e.g. 10 M.OMEGA. to produce the output of several volts at the output terminal OUT. However, when the current in voltage conversion circuit shown in FIG. 3 produces an offset current Ioff from the negative input terminal of the differential amplifier 1, the resistor 4 with a large resistance of 10M.omega. causes an offset voltage Voff of 10 volt seven if the offset current is as small as 1 .mu.A. Such a large offset voltage adversely affects the operation of the current to voltage circuit. Particularly, the problem is that the current to voltage conversion circuit which operates on a low voltage of 3 or 5 volts cannot provide a large offset voltage as well as an output signal definitely representing "1" or "0".